1. Field of the Invention
The present invention relates to an optical waveguide type filter in which a part in a longitudinal direction of an optical waveguide such as an optical fiber is formed with a periodically perturbation part having a periodic change in refractive index and the like, a method of making the same, and an optical fiber amplifier using the optical waveguide type filter.
2. Related Background Art
A long period type tilted optical fiber grating in which a part of an optical fiber is formed with a periodically perturbation part for refractive index having a relatively long period has been known from A. M. Vengsarkar, et al., xe2x80x9cLong-Period Fiber Grating as Band-Rejection Filters,xe2x80x9d JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 14, NO. 1, pp. 58-65, and the like.
On the other hand, a tilted optical fiber grating in which a part of an optical fiber is formed with a periodically perturbation part having a relatively short period such that a line perpendicular to a level plane thereof is tilted with respect to the optical axis of the optical fiber has been known from literatures such as R. Kashyap, et al., xe2x80x9cWIDEBAND GAIN FLATTENED ERBIUM FIBRE AMPLIFIER USING A PHOTOSENSITIVE FIBRE BLAZED GRATING,xe2x80x9d ELECTRONICS LETTERS, Vol. 29, No. 2, pp. 154-156; and T. Erdogan, et al., xe2x80x9cRadiation-mode coupling loss in tileted [sic] fiber phase graitings [sic],xe2x80x9d OPTICS LETTERS, Vol. 20, No. 18, pp. 1838-1840.
Both of the long period type optical fiber grating and tilted optical fiber grating are based on an optical waveguide, and function as a wavelength selection type loss filter in a wavelength band of 1.5 xcexcm and the like. When compared with an optical component having the same function, e.g., etalon, it is advantageous in that it can easily be connected to an optical waveguide such as an optical fiber while its insertion loss is low.
A long period type optical fiber grating is one in which a part of an optical fiber having a photosensitive dopant is irradiated with ultraviolet rays by use of a mask plate having a masking period of several hundreds of micrometers, so as to form the optical fiber with a refractive index perturbation part having a period of several hundreds of micrometers.
When a coating layer is disposed on the optical fiber in the long period type optical fiber grating, a leaky mode, which is formed on the whole cladding and is essential for exhibiting a function as a wavelength selection type loss filter, changes/disappears, thereby altering its cutoff spectrum. Therefore, the coating layer is hard to provide. When no coating layer is provided, however, the fear of damaging and breaking the optical fiber increases so much that its handling is not be easy. Also, the cutoff center wavelength in the long period type optical fiber grating is greatly influenced by its core/cladding refractive index difference. Since the core/cladding refractive index difference greatly varies depending on temperature, the cutoff center wavelength will vary if temperature changes.
On the other hand, the tilted optical fiber grating is considered to be in a mode more preferable as a wavelength selection type loss filter, since it does not have such demerits of the long period type optical fiber grating.
FIGS. 1A, 1B and 1C are views showing an example of tilted optical fiber grating (hereinafter referred to as xe2x80x9ctilted FGxe2x80x9d), in which FIG. 1A is a longitudinal sectional view, FIG. 1B is a lateral sectional view, and FIG. 1C is a perspective view. In FIGS. 1A, 1B and 1C, 1 is an optical fiber, 2 is a core, 3 is a cladding, 4 is a periodically perturbation part for refractive index, A is a line passing a given point O on the optical axis in the periodically perturbation part and being perpendicular to a level plane L passing the given point, X is the optical axis, Y is a deflection angle direction, L is a level plane, M is a plane, also referred to as a deflection angle plane, formed between the line A passing the given point O on the optical axis in the periodically perturbation part and being perpendicular to the level plane passing the given point and the optical axis X, O is the given point, and xcex8 is the angle of inclination.
This tilted FG is one in which a part in a longitudinal direction of the optical fiber 1 comprising the core 2 and cladding 3 is formed with a part whose refractive index is periodically changed, i.e., periodically perturbation part 4. A plane yielding a fixed refractive index in the periodically perturbation part 4, i.e., the level plane L, is tilted from a plane perpendicular to the optical axis X of the optical fiber 1. Also, the line A perpendicular to the level plane L passing a given point O in the periodically perturbation part of the tilted FG is tilted with respect to the optical axis X, whereby the line A and the optical axis X form the angle of inclination xcex8.
Within the deflection angle plane M, a direction passing the point O and being perpendicular to the optical axis X is defined as the deflection angle direction Y. Therefore, all of the optical axis X, line A, and deflection angle direction Y are located within the polarization angle plane M.
In the conventionally known tilted FG, all the level planes L are parallel to each other even when the position of given point O varies within the periodically perturbation part 4. Therefore, even when the position of given point O in the periodically perturbation part 4 changes, the deflection angle plane M is fixed, and the deflection angle directions Y are always parallel to each other and oriented in the same direction.
Such a tilted FG is made as follows. FIGS. 2A and 2B are views showing a major part of a manufacturing method, in which FIG. 2A is a perspective view, whereas FIG. 2B is a side view. In FIGS. 2A, 2B and 2C, 5 is a phase grating mask, 6 is a grating surface, 7 is an excimer laser, and 8 is an ultraviolet ray. The optical fiber 1 including a photosensitive dopant such as germanium in the core 2 is arranged parallel to the phase grating mask 5 formed with the grating surface 6 made of several thousands to several tens of thousands of groove-like recesses/projections usually having a pitch of about 1 xcexcm therebetween, and is irradiated with the ultraviolet rays 8 by way of the phase grating mask 5 by using the excimer laser 7. As a consequence, interference fringes of the ultraviolet rays 8 are generated by the grating surface 6 of the phase grating mask 5, and the optical fiber 1 is irradiated with the interference fringes. As the ultraviolet light source, not only the excimer laser but also argon lasers may be used.
Since the refractive index of the core 2 in the optical fiber 1 varies depending on whether the ultraviolet rays are strong or weak, the interference fringes of ultraviolet rays form the periodically perturbation part 4 for refractive index in the optical fiber 1. A normal optical fiber grating whose optical axis is perpendicular to the level planes of the periodically perturbation part 4 is obtained when the direction of grooves in the grating surface 6 is oriented in a direction perpendicular to the optical axis X of the optical fiber 1, whereas a so-called tilted FG in which a line perpendicular to the level planes of the periodically perturbation part 4 is tilted with respect to the optical axis is obtained when the direction of grooves of the grating surface 6 is tilted by about 5 degrees with respect to a direction perpendicular to the optical axis X of the optical fiber 1.
As mentioned above, using the tilted FG as a wavelength selection type loss filter is advantageous in that fluctuations in the cutoff center wavelength with respect to temperature changes are smaller than those in the long period optical fiber grating, and that its handling is easy since the coating layer can be provided on the optical fiber.
In the periodically perturbation part of the tilted FG in accordance with the prior art, however, the deflection angle direction is always oriented in a fixed direction, whereas its cutoff characteristic varies between the polarization in the deflection angle direction and the polarization in a direction perpendicular to the deflection angle direction, whereby its cutoff characteristic as a tilted FG is dependent on polarization. The dependence on polarization may be problematic in that, when the tilted FG is used in an optical fiber amplifier or the like, its gain may vary depending on the state of polarization of signal light.
The present invention provides an optical waveguide type filter using a tilted FG whose dependence on polarization is lowered, a method of making the same, and an optical fiber amplifier using the optical waveguide type filter.
In the optical waveguide type filter of the present invention, a part of an optical waveguide in a longitudinal direction thereof is provided with a periodically perturbation part for a periodic change in refractive index or the like, and a line perpendicular to a level plane of the periodically perturbation part is tilted with respect to the optical axis of the optical waveguide. Also, when made linear without twisting, the optical waveguide includes a portion where a plane formed by a line passing a given point on the optical axis in the periodically perturbation part and being perpendicular to a level plane passing the given point and the optical axis, i.e., deflection angle plane, varies depending on a position of the given point in the longitudinal direction thereof, whereby a part with a varied deflection angle direction is made in the longitudinal direction of the optical waveguide. As a consequence, the difference between respective cutoff characteristics with respect to the polarization in the deflection angle direction and the polarization in a direction perpendicular to the deflection angle direction is canceled in the longitudinal direction of the optical waveguide, whereby the dependence of cutoff characteristic on polarization is lowered.
The method of changing the deflection angle direction depending on the position in the longitudinal direction of the optical waveguide includes one comprising the steps of initially forming a tilted FG whose deflection angle direction does not vary in the longitudinal direction of the optical waveguide, and then twisting it about the optical axis of the optical waveguide so as to change the deflection angle in the longitudinal direction of the optical waveguide; and one comprising the steps of initially twisting a part of the optical waveguide about the optical axis, forming a tilted FG whose deflection angle direction does not vary in thus twisted part, and then untwisting it so as to change the deflection angle direction in the longitudinal direction of the optical waveguide.
An optical waveguide type filter whose deflection angle direction varies in the longitudinal direction can also be formed by a method comprising the steps of forming a plurality of tilted FGs whose deflecting angle direction does not vary in the longitudinal direction of the optical waveguide, rotating them about the optical axis with varied rotating angles such that their deflection angle directions differ from each other, and fusion-splicing them to each other so as to yield a single optical waveguide.
Further, an optical waveguide type filter whose deflection angle direction varies in the longitudinal direction can also be formed by a method comprising the steps of forming a part in the longitudinal direction of the optical waveguide with a periodically perturbation part whose deflection angle direction does not vary, forming a periodically perturbation part having a deflection angle direction different from that of the former periodically perturbation part by rotating the optical waveguide about the optical axis at a location longitudinally separated from the former periodically perturbation part, and repeating such an operation for a plurality of times, so as to form a plurality of periodically perturbation parts having respective deflection angle directions different from each other at respective locations in the longitudinal direction of the optical waveguide.
The optical waveguide type filter in accordance with the present invention formed as in the foregoing may be inserted in a circuit of an optical fiber amplifier having, at least, an erbium-doped optical fiber and a pumping laser light source, so as to act as a gain equalizer, whereby amplification spectral characteristics can be flattened in a large wavelength width.
The optical waveguide type diffraction grating device in accordance with another aspect of the present invention is characterized in that, in N refractive index modulated parts, respective lines perpendicular to refractive index level planes form the same angle with the optical axis of the optical waveguide, respective forming areas have the same length along the longitudinal direction of the optical waveguide, respective refractive index modulation periods are the same, and respective refractive index modulation amplitudes are the same. The method of making an optical waveguide type diffraction grating device in accordance with another aspect of the present invention is characterized in that the N refractive index modulated parts are formed such that respective lines perpendicular to refractive index level planes form the same angle with the optical axis of the optical waveguide, respective forming areas have the same length along the longitudinal direction of the optical waveguide, respective refractive index modulation periods are the same, and respective refractive index modulation amplitudes are the same. The optical waveguide type diffraction grating device becomes one whose polarization-dependent loss is efficiently reduced in this case as well.
The optical waveguide type diffraction grating device in accordance with another aspect of the present invention is characterized in that a polarization-dependent loss at a wavelength yielding the maximum transmission loss is not greater than {fraction (1/10)}of the maximum transmission loss value. In this case, the optical waveguide type diffraction grating device is favorably used as an optical apparatus (or a part thereof) which is required to have a low polarization-dependent loss in the field of optical communications.
The method of making an optical waveguide type diffraction grating device in accordance with another aspect of the present invention is characterized in that each of the N refractive index modulated parts is formed while monitoring a transmission loss. Alternatively, it is characterized in that each of the N refractive index modulated parts is formed while monitoring a polarization-dependent loss. In this case, the optical waveguide type diffraction grating device made thereby becomes one whose polarization-dependent loss is efficiently reduced.